The user-friendliness of user-interfaces is one of the key items that govern the acceptance and spread of data processing systems throughout the consumer and professional market segments. In this respect, the graphical user-interfaces such as touch tablets and graphical tablets have become increasingly more important, since "a picture says more than a thousand words". For background information, see European patent application EP-A 0 421 025 (U.S. Pat. No. 5,231,381; PHN 13,100) that discloses data entry systems comprising a plurality of graphical user-interfaces that are functionally and physically integrated with one another and with an LCD.
European patent application EP-A 0 307 667 discloses a coordinate input system wherein the tablet includes a plurality of electrically conductive loops for electromagnetic coupling to an LC tuning circuit in the stylus. Each loop in the tablet is alternately generating an electromagnetic wave of a fixed frequency and thereupon serving as an antenna for detection of the response of the stylus. The wave transmitted by the loop excites a tuning circuit in the stylus, thereby inducing a voltage of a particular frequency. The particular frequency is determined by a force exerted on the tip of the stylus when contacting the tablet. The tuning circuit in turn generates a second wave that is picked up by the loop. The particular frequency of the second wave is a measure of the exerted force. Upon detection by the tablet, the particular frequency can be used to control the size of a cursor shown on the tablet or the thickness of the strokes shown as a result of the data entry.
The tuning circuit includes a coil, which is connected to a capacitor to form an LC circuit. A ferrite core that is moveable into the coil is physically integrated with the tip of the stylus. A force applied to the tip of the stylus causes the tip to move axially with regard to the stylus. The position of the core relative to the coil effectively determines the inductance of the coil and, as a result the frequency of the oscillation set up in the LC circuit.